Currently, satellite positioning systems, such as GPS, are one of the most accurate location data sources available to portable or mobile electronic devices. However, there are a number of drawbacks associated with satellite positioning systems. For example, it might not be possible to receive signals from the satellites when the device is indoors, under heavy foliage or in an ‘urban canyon’ (i.e. between a number of tall buildings), making it impossible to obtain a position measurement (sometimes referred to as a ‘fix’). Satellite positioning systems can also be prone to errors in the position measurement which can be due to a number of different reasons, including ‘multipathing’ where the signals from a satellite can reflect off of buildings before reaching the satellite positioning system receiver. These errors can cause the reported position to be some distance from the actual position, sometimes even as far as several city blocks. These erroneous measurements are sometimes referred to as ‘outliers’.
GPS-enabled devices can be used to track the location of a user carrying the device. Users of such devices can include elderly people, children, people with Alzheimer's disease or dementia (who are prone to wandering) or patients in a care home or hospital. In some of these cases, ‘geofences’ can be established that bound safe or acceptable areas in which the user is allowed to move freely (or conversely areas that the user should not enter). Any obtained position measurement is compared to the geofence, and if the user is found to be outside the geofence (i.e. in an area that they should not be in), an alarm can be triggered or an alert raised. The position measurements obtained by the device can be provided to a remote location, such as a call centre or emergency services personnel, to enable the device and thus the user to be found.
It will be appreciated that in geo fencing applications, outlier position measurements can indicate that the user is outside the acceptable area and can result in an alarm being triggered, when in fact the user is within the acceptable area. The alarm can be provided for the benefit of the user (i.e. the alarm can be audible to the user of the device to indicate that they have crossed the geofence), or the alarms can be used to initiate the dispatch of people or the emergency services to locate and help the user of the device. Likewise, outlier position measurements can indicate that the user is within the acceptable area when in fact the user has crossed the geofence, which means that an alarm might not be triggered. Therefore, it is desirable to minimise false alarms caused by outliers.
Even if an alarm is correctly triggered (i.e. the user has crossed the geofence), outlier measurements relayed to the emergency services or other person dispatched to find the user can send that person off in the wrong direction and delay the assistance being provided to the user.
Therefore, there is a need for an improved technique to identify outlier position measurements.